1. Field of the Invention
The present invention relates to an improved fuse tube for a fuse, and more specifically, to an integrally molded fuse tube for a fuse or fuse cutout which is convenient to manufacture and which exhibits high burst strength along with good arc-extinguishing properties. The present invention constitutes an alternative to the invention described and claimed in commonly-assigned, co-pending United States Patent Application, Ser. No. 73,667, filed Sept. 10, 1979 in the name of Tobin, and an improvement over the following U.S. Pat. Nos. 4,104,604, issued Aug. 1, 1978 to George; 3,983,525, issued Sept. 28, 1976 to Healey; 3,922,385, issued Oct. 7, 1975 to Blewitt, Cameron and Vondracek; 3,846,727, issued Nov. 5, 1974 to Harmon; 3,120,594, issued Feb. 4, 1964 to Russel; 3,111,567, issued Nov. 19, 1963 to Stewart and Bulloch; 2,929,900, issued Mar. 22, 1960 to White; and 2,826,660, issued Mar. 11, 1958 to Kozacka.
2. Prior Art
Fuse tubes for use in fuses, fuse cutouts or similar devices are well-known. Typically, as exemplified by the Russel, Stewart and White patents, above, such fuse tubes include an outer, mechanically strong and weather resistant tube within which is located an inner sleeve made of, or containing, an ablative arc-extinguishing material such as horn fiber, melamine or the like. The outer tube and the inner sleeve are typically formed of different materials and the sleeve defines an internal bore. In typical use, conductive end fittings are mounted to the fuse tube, which is placed in a mounting. A fuse link, which includes a fusible element mounted between a stationary and a movable terminal, is located within the bore of the sleeve and the ends of the fuse link are connected to opposed points of a circuit via the end fittings. The fuse or fuse cutout operates when, upon the melting or fusing of the fusible element due to an overcurrent in the circuit, an arc is established between the terminals within the bore of the sleeve. The arc is elongated by movement of the movable contact, and the elongating arc interacts with the sleeve to rapidly evolve large quantities of deionizing, cooling and turbulent arc-extinguishing gas. Arc elongation and the action of the gas ultimately extinguish the arc and interrupt current in the circuit.
The outer tube must be mechanically strong and weather resistant for several reasons. Since fuses and fuse cutouts may reside in outdoor environments for substantial periods of time, the outer tube must be able to resist the effects thereof, including the effects of thermal cycling, rain, wind, pollution, and sunlight. Additionally, the outer tube must be mechanically strong, both to protect the fuse link and to resist bursting during operation of the fuse or cutout, during which the arc-extinguishing gases are generated. In order to ensure effective extinguishment of the arc, the fuse tube must remain integral and not burst; should the fuse tube burst, the arc may not be extinguished to the detriment of the circuit the fuse or cutout is designed to protect, as well as to the possible detriment of surrounding structures which may be damaged by a prsistent arc.
The responses of the prior art to these requirements for a fuse tube have been similar. The Russell, Stewart, and White patents, noted above, disclosed fuse tubes wherein the bore wall of the inner sleeve of the fuse participates in arc extinguishment, as discussed earlier. The fuse tube includes an ouer tube which has high burst strength and includes a thermosetting resin in which fibrous material may be included to that end. The strong outer tube is lined with the separate inner sleeve formed of, or containing, an arc-extinguishing material. The inner sleeve is inserted into the outer tube and the two are bonded together in a variety of ways. Tests of these and similar prior art fuse tubes have indicated that the effects of outdoor environments, including the effects of thermal cycling, often cause delamination or separation between the outer tube and the inner sleeve. This delamination or separation is undesirable inasmuch as both the strength characteristics of the fuse tube may be compromised and the bore of the sleeve which houses the fuse link may assume an irregular configuration which can deleteriously effect the ability of the fuse tube to effect arc extinguishment. As shown in Russell, the arc-extinguishing material inner sleeve may be rather thin. Accordingly, following a number of operations of the fuse cutout in which the fuse tube is included, the sleeve may ultimately be completely eroded in places, leaving thereat only the non-arc-extinguishing material of the outer tube exposed to subsequent arcs.
Numerous other patents exemplified by the George, Healey, Harmon, Kozacka, and Blewitt patents show fuse housings of a type specifically adapted for use in current-limiting fuse. Housings of the latter patents are made of thermosetting or similar polymeric materials which contain therein strengthening materials, such as fiberglass mat or cloth or woven or wound fiberglass strands, to improve the burst strength thereof. These housings are, however, not suitable for use as the fuse tube of non-current-limiting fuses or fuse cutouts. Specifically, all of these patents show a fuse housing having an internal cavity, the fiberglass or other strengthening material being incorporated throughout the entire or most of the thickness of the housing. Typically, the housing does not participate in arc-extinguishment. Should such a housing be used in a fuse cutout, even if the interior of the walls of the housing have arc-extinguishing properties, which they do not, after only a very few uses of the fuse tube, sufficient material would be ablated from the interior of the housing by arcing to expose the fiberglass or other strengthening material. This exposed material both does not have arc-extinguishing properties and, due to the creation of internal surface irregularities within the tube, could degrade or interfere with the arc-extinguishing action such a fuse tube should exhibit.